Inflammatory bowel diseases (IBD) are the result of interactions between susceptibility and severity genes, the environment and the Innate and adaptive arms of the mucosal immune system. Results from several disease- and/or antibody-based genome-wide association studies have pointed to variants in several genes important in innate and adaptive immune responses related to Crohn's disease (CD) risk, protection and severity. Data generated over the last cycle of this PPG demonstrate that In CD and ulcerative colitis (UC), Tumor necrosis factor superfamily member-15 (TNFSF15) is associated with risk, protection and severity and toll like receptor (TLR)-8 is associated with risk and protection. We have shown an interaction between TLR8 signaling and expression of the TNFSF15 associated protein, TLIA. Activation of TLR8 signaling inhibits TLIA expression, which Is In contrast to its known function of inducing other pro- and anti-inflammatory molecules. However, there is great variability among controls and CD patients in the level of TLR8 as well the degree of TLR8 inhibition of TLIA surface and soluble mRNA expression that may be related to different variants in the TNFSF15 and TLRS genes. We have shown that TLIA is a central regulator of Th1 and Th17 mucosal inflammation in the mouse and in severe human CD, and that TLR8 is so far the only pathway capable of down regulating TLIA. Therefore, studying the functional consequences of IBD associated variants in these genes on in vitro TLIA expression and in vivo TLR8 regulation of TLIA-dependent and -independent mucosal inflammation is significant. This continuation of Project 2 is based on the following hypothesis: Patients with genetic variants in risk or severity associated haplotypes of TNFSF15 combined with those of TLR8 lead to enhanced expression of TLIA which is poorly inhibited when TLRS signaling is defective. Furthermore, patients with combinations of gene variants associated with high level TLIA expression and less TLRS inhibition have the greatest expression of TLIA in the mucosa and will have the most severe inflammation/clinical course. Specific Alms: 1) Determine the in vitro functional consequences of CD-related genetic variants on TLRS and TNFSF15; 2) Determine the in vivo effect of endogenous and exogenous ligand signaling of TLR8 In TLIA-dependent and -independent mucosal Inflammation; and 3) Determine the relationship of combinations of TNFSF15 and TLR8 genetic variants to clinical subtypes and how these variants relate to natural history.